In this paper, a modified stochastical interparticle collision model is discussed and valdiated. A channel flow with a constriction serves as a test case. The simulation of the thereby colliding particle strands clearly shows that without the collision sub-model, the simulation with the standard Lagrangian model yields unreasonable results. After adding the stochastical collision sub-model, the Lagrangian model is able to describe the flow situation if the mass load is not too high.